This invention relates to a process of purifying flue gases or other humid exhaust gases which contain SO.sub.2, NO.sub.x and other gaseous polluants, wherein the SO.sub.2 content is oxidized to SO.sub.3 and the NO.sub.x content is reduced by a catalytic treatment, the SO.sub.3 content is condensed as sulfuric acid and other gaseous polluants are removed by being scrubbed with aqueous liquids at low temperatures.
Flue gases from fuel-firing furnaces contain polluants consisting mainly of SO.sub.2, NO.sub.x, HCl and HF. For instance, the combustion of Ruhr coal which contains 1% sulfur by weight in large furnaces will result in a flue gas which contains 6% by volume oxygen and contains per sm.sup.3 between 1800 and 2000 mg SO.sub.2, below 150 mg HCl and below 50 mg HF. In dependence on the combustion process and the coal employed the NO.sub.x content will range from less than 600 to and above 2000 mg NO.sub.2 /sm.sup.3.
It is known that the flue gas can be catalytically aftertreated in order to decrease the emission of such polluants. By that aftertreatment the SO.sub.2 is oxidized to SO.sub.3 and the NO.sub.x is reduced with NH.sub.3 to N.sub.2. The humid gas is subsequently cooled below the dew point temperature of sulfuric acid and the SO.sub.3 is condensed as sulfuric acid.
Such a process is known from Published German application No. 33 31 545. The SO.sub.2 -containing hot flue gas is adjusted to the temperature which is required for the catalysis and is subsequently catalytically reacted. The SO.sub.3 -containing humid gas is cooled in a first stage of an air preheater to a temperature above the dew point temperature of sulfuric acid and is subsequently cooled in a second stage below the dew point temperature of sulfuric acid. The second stage of the air preheater consists of an acid-resisting material, such as glass tubes. In the second stage the gas cannot be cooled below a temperature with which the minimum chimney inlet temperature prescribed in the regulation for large fuel-firing plants and the required lift of the exhaust gas in the chimney are still ensured. As a result, the gas cannot be subjected to a fine purification at low temperatures.
U.S. Pat. No. 4,164,546 describes such a process in which the hot flue gases are catalytically treated with an addition of ammonia. After the catalysis the SO.sub.3 content may be removed by means of an aqueous absorbent solution. During the absorption the gas must not be cooled below the required minimum chimney inlet temperature of the gas.
Published German application No. 36 01 378 describes a process in which the hot flue gases are catalytically treated with an addition of ammonia. After the catalysis and an interstage cooling, the SO.sub.3 is reacted with water to form sulfuric acid and the purified gas is delivered to the chimney. In that case too the exit temperature from the SO.sub.3 absorber must correspond to the required chimney inlet temperature.
In the unpublished German patent application No. P 36 24 462.7 a process has been described in which the hot flue gases are also adjusted to a suitable temperature before the catalysis. After the catalysis the flue gas is cooled in a heat exchanger by an indirect heat exchange to a temperature below the dew point temperature of sulfuric acid. The cooled gas is cooled further to a predetermined temperature by sprayed dilute sulfuric acid in a countercurrent operation, and a dilute sulfuric acid having a predetermined concentration is condensed. The gas exiting from the SO.sub.3 condenser is scrubbed in a fine scrubber by a sprayed aqueous liquid and is thus cooled to 40.degree. to 55.degree. C. The gas exiting from the fine scrubber is reheated in a reheater to a predetermined temperature by sprayed dilute sulfuric acid. The gas from which SO.sub.3 has been condensed is passed through a filter. Dilute sulfuric acid from the reheater is sprayed into the SO.sub.3 condenser. The temperature of the gas in the reheater is so selected that the water that has been absorbed in the scrubber will not condense and the temperature of the gas exiting from the heat exchanger is adjusted approximately to such a temperature that the temperature of the dilute sulfuric acid obtained in the SO.sub.3 condenser is approximately the temperature of the gas in the reheater. The dilute sulfuric acid is sprayed into the reheater at such a rate that the dilute sulfuric acid drained from the reheater is approximately at the required gas exit temperature from the SO.sub.3 condenser.
In that process the gases can be subjected to a fine purification at low temperatures after the catalysis because the gases can subsequently be reheated to the required minimum chimney inlet temperature. But before the catalysis the flue gases must be at the temperature which is required for the catalysis.
In many cases, e.g., when dust has previously been collected at lower temperatures, the temperature of the flue gases before the catalysis is too low for the catalysis.
It is an object of the invention to permit a catalytic treatment of relatively cold and substantially dustfree flue gases and a subsequent recovery of sulfuric acid and fine purification in a most economical manner.